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Tight Turns

Tight Turns
Rifling twist rates have been changing due to longer and longer bullets.

Jim Carmichel, whose wisdom I trust in all things involving the rifle, once wrote, “No shooting subject is more likely to make one sound like an expert, and at the same time prove him a fool, than a discussion of rifling twist.” And since Jim went on to say, “I hope I am quoted on this,” I’m happy to do so, and in turn, hope I can avoid sounding foolish.

Twist is generally measured in terms of distance traveled for one revolution of the projectile, for example one turn in ten inches, usually written as 1:10″. Twist can also be expressed in terms of calibers (e.g. a “40-caliber twist,” or in the angle of the rifling grooves relative to the bore axis, but we’ll stick with rotation/distance terms.

The purpose of rifling, of course, is to impart spin to the projectile so it follows a more consistent and predictable path. A projectile with little or no spin tends to go straight for a short distance, then move about unpredictably—which is why in baseball, when a knuckleballer is on the mound pitching, the catcher wears an oversize catcher’s mitt.
A sphere is relatively easy to stabilize, so early muzzle-loading rifles had twists such as 1:66″. The longer a projectile is, the faster it must be spinning to stabilize. Since heavier bullets are usually longer we tend to equate spin with bullet weight, though in fact bullet length is what matters.

Bullet stability depends on how rapidly the bullet is spinning. Rate of spin depends on two factors: rifling twist, and bullet velocity.

Compare two 0.224″ barrels, both with a 1:12″ twist, both firing identical 50-grain bullets. One is chambered in .222 Rem, with 3,000 fps muzzle velocity. At one turn per foot, and going 3,000 fps, it exits the bore spinning at 3,000 revolutions per second, 180,000 rpm.

The other is a .220 Swift with muzzle velocity of 4,000 fps. The bullet exits spinning at 4,000 revolutions per second, or 240,000 rpm. If we want the same rate of spin from our .222 Rem, we’ll need a barrel with a 1:9″ twist, which at a muzzle velocity of 3,000 fps spins the bullet at 240,000 rpm.

If a bullet isn’t spinning fast enough the result is obvious; bullets yaw until they lose all semblance of stability and accuracy, tumble end for end, and hit the target flying sideways if they hit it at all.
The negative effects on a bullet spinning faster than necessary are less obvious; in fact there may not be any. If the bullet is out of balance, spinning it faster just makes things worse, though with today’s bullets you have to search hard to find a bad one.

A thin-jacketed bullet spun too fast may simply fly apart in flight. Now there’s a fairly obvious effect, though one a shooter can easily avoid by selecting bullets appropriate to velocity.

A faster twist opens more doors than it closes. It gives the shooter the option to use longer bullets. Any loss of accuracy with shorter bullets is generally either nonexistent or so small as to be important only to benchrest competitors.
For a long time riflemen firmly believed rate of twist should be just fast enough to stabilize the heaviest bullets which were likely to be used, and no faster. Factory rifles often had slower twists, for example 1:14″ in the .250 Savage, 1:12″ in the .244 Rem.
In recent years the trend has been to faster twists, for several reasons:

• Spitzer bullets are longer than roundnose bullets.
• Monometal bullets are less dense than lead-core bullets, and are longer for the same bullet weight.
• Ballistically efficient very-low drag (VLD) bullets are longer than conventional spitzer bullets.
If your rifle has a slower twist but does what you want, just keep on using it. I’m not going to quit shooting my various .222 to .220 Swift rifles just because they have 1:12″ and 1:14″ twists.

Twist rates for most current cartridges are adequate, but a few could use improvement. In a new .223 Rem or .243 Win rifle I want at least a 1:9″ twist. If you’re planning to use some of the heavier VLD bullets (75 grains in 0.224″ or 105 grains in 0.243″) a 1:8″ is a better choice. Any .223 Rem rifles I’ve bought recently have 1:8″ or 1:9″ twists.
In .243 Win I want at least a 1:9″ twist. I’d take a 1:8″ if it was offered or if I was installing a custom barrel. The other cartridge worth noting is the .308 Win. A lot of rifles for this cartridge have a 1:12″ twist. For use with heavier VLD bullets (175 grains and up) a 1:10″ would be better.
By Dave Anderson

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